Vast crowds of Hindu pilgrims in India began bathing in sacred waters on Monday as the Kumbh Mela festival opened, with organisers expecting 400 million people -- the largest gathering of humanity.

The millennia-old Kumbh Mela, a show of religious piety and ritual bathing -- and a logistical challenge of staggering proportions -- is held at the site where the Ganges, Yamuna and mythical Saraswati rivers meet.

In the cool pre-dawn gloom, pilgrims surged forward to begin bathing in the waters.

"I feel great joy," said Surmila Devi, 45.

"For me, it's like bathing in nectar."

Businesswoman Reena Rai's voice quivered with excitement as she spoke about the "religious reasons" that brought her to join the sprawling tents, packed along the river banks in the north Indian city of Prayagraj in Uttar Pradesh state.

"As a Hindu, this is an unmissable occasion," said the 38-year-old, who travelled around 1,000 kilometres from Madhya Pradesh state to take part in the festival, which runs from Monday until February 26.

Saffron-robed monks and naked ash-smeared ascetics roamed the crowds offering blessings to devotees, many of whom had walked for weeks to reach the site.

The massive congregation is also an occasion for India's Hindu nationalist government to burnish its credentials.

Prime Minister Narendra Modi called it a "divine occasion", that brings together "countless people in a sacred confluence of faith, devotion and culture".

Yogi Adityanath, a Hindu monk and Utter Pradesh's chief minister, welcomed devotees to "experience unity in diversity" at the "world's largest spiritual and cultural gathering".

Organisers say the scale of the Kumbh Mela is that of a temporary country -- with numbers expected to total around the combined populations of the United States and Canada.

"Some 350 to 400 million devotees are going to visit the mela, so you can imagine the scale of preparations," festival spokesman Vivek Chaturvedi said.

Hindu monks carried huge flags of their respective sects, while tractors turned into chariots for life-size idols of Hindu gods rolled by behind them accompanied by elephants.

Pilgrims exulted in the beat of drums and honking horns.

The festival is rooted in Hindu mythology, a battle between deities and demons for control of a pitcher containing the nectar of immortality.

Organising authorities are calling it the great or "Maha" Kumbh Mela. (AFP)